This invention relates to measuring instruments. More particularly, the invention relates to an instrument for measuring strain in structural members, incorporating a diffractive mechanical grating device, that is particularly suitable for a wide range of environmental conditions, including both cryogenic and high temperatures.
Advances in micromachining technology have given rise to a variety of micro-electromechanical systems (MEMS) including mechanical grating devices for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. Representative examples of these modulators are disclosed in U.S. Pat. No. 4,492,435 to Banton et al for a xe2x80x9cMultiple Array Full Width Electromechanical Modulatorxe2x80x9d; U.S. Pat. No. 4,596,992 to Hornbeck for a xe2x80x9cLinear Spatial Light Modulator and Printer; U.S. Pat. No. 5,311,360 to Bloom et al for xe2x80x9cMethod And Apparatus For Modulating a Light Beamxe2x80x9d; U.S. Pat. No. 5,611,593 to Engle for a xe2x80x9cLinear Electrostatic Modulatorxe2x80x9d; U.S. Pat. No. 5,757,536 to Ricco et al for an xe2x80x9cElectrically Programmable Diffraction Gratingxe2x80x9d; commonly-assigned U.S. Pat. No. 6,038,057 to Brazas, Jr. et al. for xe2x80x9cMethod and System For Actuating Electro-mechanical Ribbon Elements In Accordance to a Data Streamxe2x80x9d and commonly-assigned U.S. Pat. No. 6,061,166 to Furlani et al for a xe2x80x9cDefractive Mechanical Grating Devicexe2x80x9d. Micromachined mechanical grating devices are of particular interest and versatility for strain gauge applications.
Other MEMS devices have been used to sense various physical properties such as acceleration, pressure, mass flow, temperature, humidity, air density or weight. Representative devices are disclosed in U.S. Pat. No. 5,090,254 to Guckel et al for xe2x80x9cPolysilicon Resonating Beam Transducersxe2x80x9d; U.S. Pat. No. 5,275,055 to Zook and Burns for a xe2x80x9cResonant Gauge With Microbeam Driven In Constant Electric Fieldxe2x80x9d; U.S. Pat. No. 5,417,115 to Bums for xe2x80x9cDielectrically Isolated Resonant Microsensorsxe2x80x9d; and U.S. Pat. No. 5,550,516 to Burns and Zook for Integrated Resonant Microbeam Sensor and Transistor Oscillatorxe2x80x9d. The sensors disclosed in these patents are said to operate on the principal that the natural frequency of vibration (i.e. resonate frequency of an oscillating beam or other member) is a function of the strain induced in the member. More particularly, tensile forces tending to elongate the member and increase its resonate frequency, while forces tending to compress the member and reduce its natural frequency. The dual vibrating beam transducers disclosed in U.S. Pat. No. 4,901,586 to Blake et al are said to operate in an apparently similar manner. All of the above mentioned transducers and sensors use integrated electrical means to sense the motion of the moving member. This limits the design and placement of both the sensors and the associated electronics.
It is an object of the present invention to provide an optical strain gauge which can provide highly accurate measurements of the strain in a structural member.
It is another object of the present invention to provide an optical strain gauge in which the strain in a structural member can be monitored at a distance away from the structural member.
These objects are achieved by an optical strain gauge for measuring the strain in a structural member comprising:
(a) a mechanical grating device fixedly attached to the structural member for modulating an incident beam of light by diffraction;
(b) at least one source of light;
(c) an optical system for directing light onto the mechanical grating device and a sensor for receiving light reflected from the mechanical grating device for producing an output signal;
(d) the mechanical grating device including:
(i) an elongated ribbon element including a light reflective surface, such elongated ribbon element having a predetermined resonant frequency;
(ii) a substrate and a pair of end supports for supporting the elongated ribbon element at both ends over the substrate;
(iii) at least one intermediate support between the end supports so that there are suspended portions of the elongated ribbon element; and
(iv) a drive circuit for applying a force to the elongated ribbon element to cause the suspended portions of the elongated ribbon element to deform at the resonant frequency between first and second operating states;
(e) output circuitry responsive to the output signal produced by the sensor for extracting a frequency dependent signal which represents the strain in the structural member that caused a variation in the resonant frequency; and
(f) an output device responsive to the extracted frequency dependent signal for producing a representation of the strain in the structural member.
The present invention provides an optical strain gauge with at least one optical sensor that can be remotely positioned with respect to the structural member. The preferred embodiment includes a light source that provides light of a wavelength xcex, one or more light sensors, a mechanical grating device, and an optical system for directing and focusing light from the light source onto the mechanical grating device and directing the modulated light to the light sensor(s), output circuitry responsive to the output signal produced by the sensor for extracting a frequency dependent signal which represents the strain in the structural member that caused a variation in the resonant frequency, and an output device responsive to the extracted frequency dependent signal for producing a representation of the strain in the structural member. The mechanical grating device is designed to modulate incident light having a wavelength xcex. It includes an elongated ribbon element having a light reflective surface; a pair of end supports for supporting the elongated ribbon element at both ends over a substrate; at least one intermediate support between the end supports; and means for applying a force to the elongated ribbon element to cause the elongated ribbon element to deform between first and second operating states.
One advantage of the optical strain gauge of the invention is that the strain of a structural member can be sensed at locations remote from the structural member. Another advantage is the sensitivity of the optical strain gauge due to the size of its features. Specifically, the optical strain gauge features are on the order of microns, and it can measure changes in length on the order of nanometers. Other features and advantages of this invention will be apparent from the following detailed description.